The United Nations human rights chief, Navi Pillay, has said the failure of the UN Security Council to take action on Syria emboldened its government to launch what she called an all out assault to crush dissent.
She told the UN General Assembly that it appeared the Syrian army had launched indiscriminate attacks on civilian areas in the city of Homs.
Ms Pillay said she had asked the UN to request the International Criminal Court to investigate whether crimes against humanity were being committed in Syria.
Meanwhile, the US has backed the Arab League's latest plan for Syria.
However, it sees challenges in winning UN approval for peacekeepers to halt the Syrian government's violent crackdown on protests, US Secretary of State Hillary Clinton said on Monday.
Mrs Clinton discussed Syria with visiting Turkish Foreign Minister Ahmet Davutoglu.
They agreed to discuss further steps at the inaugural meeting of a new "Friends of Syria" contact group in Tunisia on 24 February.